There has been an increase in the number of patients seeking orthodontic treatment in recent years. With the ever-increasing emphasis on esthetic aspects of treatments, the demand for tooth-colored and highly esthetic brackets, such as ceramic brackets, has increased.^[1] Ceramic brackets are divided into either monocrystalline or polycrystalline types based on their crystalline structure. While monocrystalline alumina ceramics have a surface roughness comparable to that of stainless steel, polycrystalline alumina ceramics have a rougher surface due to the fact that their surface is made up of numerous crystals.^[2,3] The roughness of nonshedding surfaces in the oral cavity is directly associated with biofilm formation and plaque accumulation.^[4] As a result of biofilm formation and the difficulty of maintaining proper oral hygiene and plaque control, approximately 50% of orthodontic patients suffer dental demineralization ranging from mild to severe.^[5]

Streptococcus mutans is one of the predominant microorganisms in the oral cavity^[6] and is widely regarded as one of the main pathogens in the process of dental caries.^[7] Candida albicans is the dominant yeast in the oral cavity and the most abundant nonbacterial microorganism in the oral cavity.^[8] The investigation of adhesion and biofilm formation by these organisms can be vital for the prevention of the often-irreversible carious lesions that may compromise the esthetics of the orthodontic treatment and the oral health of the patient.

Previous studies regarding biofilm formation on ceramic brackets have conflicting results and are sparse, especially regarding the crystalline structure of the ceramics. Most of the studies have generally compared ceramic brackets with steel brackets, with only a few distinguishing between different crystalline types and associated microbial adhesion. In their study, Brusca et al.^[9] found that combined adhesion of S. mutans and C. albicans is lower in metals compared with the control, almost the same in ceramics compared with the control, and higher than the control in composites. van Gastel et al. found more bacterial colonies on ceramic brackets than conventional metal brackets.^[10] On the other hand, Lindel et al.^[11] and Saloom et al.^[12] have found lower microbial plaque adhesion on ceramic brackets compared to metal brackets. The studies conducted by Papaioannou et al.,^[13] Passariello and Gigola,^[14] Thaweboon et al.,^[15] and Lee et al.^[16] have generally found no significant difference in microbial plaque adhesion between ceramic and metal brackets. Among the aforementioned studies, the ones conducted by Lee et al., Passariello and Gigola and Thaweboon et al. have investigated both monocrystalline and polycrystalline types of ceramic brackets. Lee et al.^[16] stated that even though the surface roughness of monocrystalline brackets was lower than polycrystalline and even steel brackets, the biofilm adhesion was slightly higher in monocrystalline brackets compared to other groups, which was statistically insignificant. In the study conducted by Passariello and Gigola, it was found that the rate of biofilm formation in saliva on polycrystalline brackets was higher compared with monocrystalline brackets, but it was stated that this difference is not high enough to result in any clinically significant distinction between these two materials.^[14] Thaweboon et al.^[15] found significantly lower adhesion for S. mutans on monocrystalline brackets compared with polycrystalline ones.

Due to the mentioned controversies and conflicting results, we decided to specifically investigate the effect of the crystalline structure (monocrystalline vs. polycrystalline) of ceramic brackets on the adhesion of S. mutans and C. albicans in an in vitro environment. The results of this study would be beneficial in preventing and controlling the accumulation of these microorganisms in orthodontic patients and reducing their undesirable effects, such as dental caries.

The results consisted of 40 samples of monocrystalline and 40 samples of polycrystalline brackets, each divided into two groups: S. mutans alone, and S. mutans in addition to C. albicans.

The qualitative and quantitative results of the adhesion test can be found in Tables 2 and 3. Since the adhesion scores across groups did not have a normal distribution, the nonparametric Mann–Whitney test was used to compare adhesion scores. In the samples containing Streptococcus mutans, the mean adhesion score was 2.35 for the polycrystalline group and 1.85 for the monocrystalline group (27% difference), which was statistically significant (P = 0.035). In the samples containing both S. mutans and C. albicans, the mean adhesion score was higher in the polycrystalline group compared with the monocrystalline group (2.0 vs. 1.6), but this difference was not statistically significant (P = 0.108).

This study showed that the adhesion of S. mutans with or without C. albicans is lower in monocrystalline ceramic brackets compared with polycrystalline ones, even though this difference did not reach statistical significance when both organisms were present in the biofilm. The first factor contributing to this finding could be the difference in surface characteristics between these two ceramic groups. Lee et al. investigated the surface characteristics of the common materials used in orthodontic treatment and their effect on the adhesion of S. mutans.^[16] Their samples included nine different materials (four orthodontic adhesives, three bracket types, hydroxyapatite brackets, and bovine incisors). Samples were evaluated by confocal laser microscopy for differences in surface characteristics and were incubated for 3 and 6 h with and without saliva. The findings showed significant differences in surface roughness, with monocrystalline ceramics showing the lowest surface roughness, followed by metals and polycrystalline ceramics. The lowest S. mutans adhesion was observed in polycrystalline ceramics, followed by metals and monocrystalline ceramics, although these differences in adhesion were not statistically significant.^[16] A possible limitation in the extension of the results of this study is that the adhesion of biofilm was not investigated past 6 h of incubation time, even though the adhesion, accumulation, and maturation of microbial biofilm is a continuous process^[17] that continues well beyond 6 h based on the culture medium and can result in significant changes.^[18]

The aforementioned temporal limitation is addressed in the study by Passariello and Gigola, in which the in vitro adhesion of six strains of S. mutans to 15 different brackets was investigated. Each study group consisted of 12 brackets, which were incubated for 4 and 848 h at 37°C with microbial suspensions acquired from separate incubations in culture media and saliva. The results showed that the highest rate of growth of microbial biofilm adhesion among ceramics belonged to the polycrystalline brackets.^[14] In the aforementioned study, brackets were fixed onto acrylic blocks via composites and orthodontic bonds. Although their methodology could better represent the environment encountered inside the oral cavity, the effect of the materials used to fixate the brackets cannot be ignored. The surface of orthodontic bonds and composites can act as a suitable surface for bacterial adhesion,^[16] and as such, the presence of these materials in our study could alter the results. Due to these issues and since our aim was to isolate and investigate the effect of the crystalline structure of ceramic brackets on bacterial adhesion, we decided not to use this methodology. Even though this methodology could have better simulated the oral cavity, the pH of saliva, its contents, and their concentrations vary between different people, cannot be reliably controlled, and can alter the findings.

The currently available ceramic brackets are manufactured from alumina in either monocrystalline or polycrystalline form. Monocrystalline alumina has a lower surface roughness than polycrystalline alumina.^[16] Rough surfaces facilitate microorganism adhesion via increased retention and available surface. In addition, the free surface energy is higher in polycrystalline alumina compared with monocrystalline alumina.^[16] These differences in surface characteristics between polycrystalline and monocrystalline alumina can explain why S. mutans with or without C. albicans have lower adhesion to monocrystalline alumina. Higher surface roughness not only facilitates microbial adhesion but also hinders the sliding of the archwire in the bracket slot.^[2] Monocrystalline alumina has a higher surface hardness than polycrystalline forms, but if a crack develops, it can freely propagate through the entire crystal, often resulting in bulk fractures.^[19] Polycrystalline forms show lower surface hardness but higher resistance to crack propagation due to their crystalline structure, which compresses the crack line and slows its propagation.^[2] In addition to the aforementioned differences, there is also a difference in the esthetic aspect of these crystalline forms. While both forms display acceptable esthetics, monocrystalline alumina has higher translucency and can provide better esthetics, especially while observing from very close ranges.^[2]

Although conducting this study in an in vivo setting would have been closer to clinical scenarios, defining appropriate and sufficient inclusion and exclusion criteria for this type of study would have been difficult. Not only would complete matching between samples have been difficult and require very large sample sizes, but homogenizing study settings between different samples would be very hard. Factors such as food intake, diet, hygiene habits, microflora variations, the saliva and its pH, contents, and their concentrations are unpredictable and different among individuals and can affect the results. As the primary objective of this study was to investigate the effect of the crystalline structure of the ceramics on microbial adhesion, we took advantage of conducting the study in vitro to exclude potential confounders, isolate the desired interactions, and reduce potential bias.

We found that monocrystalline ceramic brackets had less overall microbial biofilm adhesion compared to polycrystalline ceramic brackets, especially when incubated with S. mutans alone. This observation might be explained primarily by lower surface roughness in monocrystalline ceramics.